Collagen and its derived products are used extensively in the production of collagen-containing implantable scaffolds. Collagen is well recognized as a material that has low antigenicity, is biodegradable and has good mechanical, haemostatic and cell-binding properties (Sheu et al., (2001), Biomaterials, 22(13):1713-9; Pieper et al., (2002), Biomaterials, 23(15):3183-92; Chvapil et al., (1973), Int Rev Connect Tissue Res., 6:1-61; Pachence (1996), J. Biomed. Mater. Res.; 33(1):35-40; and Lee et al., (2001), Int J Pharm.; 221(1-2):1-22), which enables it to be used to replace or repair tissue temporarily or permanently. Collagen scaffolds are routinely used a substrate upon which cells are able to proliferate and differentiate and being eventually replaced by normal tissue.
However, it is also well known that collagen-containing scaffolds can provoke inflammation and/or fibrosis when implanted. See, for example, Wisniewski et al., (2000), J. Anal Chem.; 366 (6-7) (p. 611-621). As a consequence, collagen-containing scaffolds are typically chemically or physically treated (cross linked) to confer mechanical strength and resistance to enzymatic (collagenase) degradation. There are several cross-linking strategies that have been used on collagen-containing materials. Glutaraldehyde is the most widely used cross-linking agent (Sheu et al., (2001) supra; Barbani et al., (1995), J Biomater. Sci. Polym. Ed.; 7(6):461-9). However, glutaraldehyde and its reaction products are associated with cytotoxicity in vivo, due to the presence of cross-linking by-products and the release of glutaraldehyde-linked collagen peptides during enzymatic degradation (Huang-Lee et al., (1990), J Biomed Mater Res., 24(9):1185-201; van Luyn et al., (1992), Biomaterials, 13(14):1017-24.
In order to avoid in vivo cytotoxicity of glutaraldehyde cross-linked collagen, several alternative compounds have been examined as potential collagen cross-linking agents (Khor (1997), Biomaterials, 18(2): 95-105; Sung et al. (1996), Biomaterials; 17(14): 1405-10) such as polyepoxy, hexamethylene diisocyanate (HMDI), 1-ethyl-3-(3-dimethylamino-propyl)carbodiimide (EDC), and ultra-violet (UV) or gamma-ray irradiation. Koob et al., (2001), J Biomed Mater Res., 56(1):31-48 showed that nordihydroguaiaretic acid (NDGA) significantly improved the mechanical properties of synthetic collagen fibres. In addition, they showed that NDGA cross-linked collagen fibres did not elicit a foreign body response nor did they stimulate an immune reaction during six weeks in vivo.
However, despite all of these advancements there remain issues with using cross-linked collagen as well as native collagen. Thus, there is still a need for a collagen-containing scaffold that has the following properties:
a) pores that interconnect in such a way as to favour tissue integration and vascularisation;
b) biodegradability and/or bioresorbability so that normal tissue ultimately replaces the scaffold;
c) surface chemistry that promotes cell attachment, proliferation and differentiation;
d) strength and flexibility; and
e) low antigenicity.
One area that has a particular need for a replacement collagen-containing tissue is the repair of tympanic membrane (TM) perforations. If left untreated, TM perforations can result in hearing loss, recurrent otorrhea, possible middle ear infection and acquired cholesteatoma (Parekh et al., (2009), The Laryngoscope; 119:1206-1213). Although most acute TM perforations heal spontaneously, large or chronic TM perforations, especially from chronic suppurative otitis media, often fail to heal and may require grafting (Lindeman et al., (1987), Archives of Otolaryngology—Head and Neck Surgery; 113:1285).
Currently, surgical methods such as myringoplasty are regarded as the most effective and reliable treatment for TM perforations (Sheehy et al., (1980), The Annals of otology, rhinology, and laryngology; 89:331; Karela et al., (2008), European Archives of Oto-Rhino-Laryngology; 265: 1039-1042). Various autologous grafts and allografts such as muscle fascia, cartilage, perichondrium and AlloDerm have been used, however, all have their own limitations (Levin et al., (2009), Expert review of medical devices; 6:653-664). For instance, temporalis fascia, which is regarded as the “gold standard”, is associated with donor site morbidity, additional incisions, long operation time and a shortage of material in revision cases (Levin et al., (2009), supra). To date, a range of xenografts and synthetic materials, including GELFOAM® membrane (Abbenhaus, (1978), Otolaryngology; 86:ORL485), paper patch (Golz et al., (2003), Otolaryngology—Head and Neck Surgery; 128: 565) and hyaluronic acid derivatives (Teh et al., (2011), Expert Opinion on Biological Therapy; 1-14) have been investigated as suitable scaffolds to support the regeneration of TM. However, there is little evidence to support any of these as optimal materials for various types of perforations.
Moreover, several commercially available xenografts such as porcine small intestinal submucosa, contain xeno DNA materials and evoke an inflammatory response due to the remnant xenocellular components including serotonin. In addition, synthetic materials are non-biodegradable, and their biomechanical and material properties are different compared to the normal TM, which may affect the long-term hearing function (Levin et al., (2009), supra). Hence, there is a constant search for better materials to achieve improved healing and hearing.